We have observed that approximately 15% and 30% of mRNA in mouse liver and a mouse ascites hepatoma, respectively, are found in cytoplasmic ribonucleoprotein complexes (mRNP), not in functioning polyribosomes. The mRNA species present in these mRNP are a distinct subset of the total mRNA. This conclusion is based on parallel studies using both nucleic acid hybridization and cell-free synthesis of proteins (displayed by two-dimensional gel electrophoresis). While the cytoplasmic free mRNP contain a much simpler population of mRNA sequences than the polysomes, the distribution of copies of the individual mRNA sequences between the functioning and non-functioning states is complex. The evidence suggests that free mRNP is not a simple precursor pool for polysomal mRNA. We intend to characterize these mRNAs in more detail and identify the proteins bound to them in order to assess their potential role in the regulation of the expression of these RNA sequences in normal and transformed cells. Conditions which influence the distribution of specific mRNAs between RNP and polyribosomes will be investigated and the molecular basis of this translational control process will be sought.